Flow Process Recognition for Floods in Mountain Streams

In Reply Refer To:                                   July 21, 1992
Mail Stop 415


Subject:  Flow Process Recognition for Floods in Mountain Streams

Whenever floods are investigated, especially in small, mountainous 
basins, one of the most important tasks is to properly identify 
the flow process that occurred in the basin.  This responsibility 
may be overlooked despite the fact that debris flows can occur in 
steep terrain throughout the country.  Incorrect identification 
and documentation of flow events can result in questionable 
discharge estimates.  Historically, some debris flows in mountain 
drainage basins have been analyzed as water floods (Costa and 
Jarrett, 1981).  As noted in Office of Surface Water Technical 
Memorandum 92.10, recent review of the peak-flow data base in 
California has emphasized concerns that some peaks computed as 
water floods were actually debris flows.  Misrepresentation of 
such peak flows could have profound impacts on flood-frequency 
estimates.  The purpose of this memorandum is to bring the 
importance of flow-process identification to the attention of 
Division personnel and to provide guidance on how to properly 
identify flow processes.  

When debris flows occur, water is mixed with sufficient volumes of 
sediment to cause water and sediment to no longer behave as two 
separate phases; they move as a single, almost plastic-like body. 
Peak discharge determinations based on traditional indirect 
measurement methods will be flawed when debris flows occur.  Not 
only will a significant amount of the cross-sectional area below 
high-water marks represent sediment rather than water, but the 
hydraulic properties of a plastic-like water-sediment mixture (a 
non-Newtonian fluid) differ significantly from those of water (a 
Newtonian fluid), on which traditional hydraulic theory is based.

The shear strength of debris-flow material results in diagnostic 
sedimentology and landforms.  Two papers are attached that 
summarize sedimentology and landforms associated with water floods 
and debris flows.  Attachment 1 is entitled, "Rheologic, 
Geomorphic, and Sedimentologic Differentiation of Water Floods, 
Hyperconcentrated Flows, and Debris Flows," by Costa (1988). 
Attachment 2 is an informal summary on "Describing and Naming 
Detrital Sediments," which was prepared by Tom Pierson in 
Vancouver, Washington, to guide field work in basins of known 
debris-flow activity.  The debris-flow process can be viewed on a 
videotape (Open-file Report 84-606, "Debris-flow Dynamics" by 
J. E. Costa and G. P. Williams (1984), 22.5 minutes) available for 
loan from any Regional Surface-Water Specialist, the National 
Training Center videotape library, or the Office of Surface Water.

In reading the attached papers, you will notice that the term 
hyperconcentrated flow is assigned to flows that are intermediate 
between clear-water floods and debris flows. It is generally 
thought that traditional hydraulic computational methods can be 
applied to hyperconcentrated flows.

Following is a synopsis of the two attachments.  A glossary of 
terms is printed at the end of this memorandum to assist readers 
who are unfamiliar with terminology used in these descriptions.  

Deposits usually associated with water flooding:

o  Deposits typically are stratified.  Beds are likely to show 
horizontal stratification, cross-bedding, and/or imbrication (see 
fig. 5 in Attachment 1 and fig. 1 in Attachment 2 ).

o  Individual beds within deposits usually are thin:  a few mm (a 
mm equals about .003 ft) to several tens of cm thick (1 cm equals 
about .03 ft).

o  Deposits are very loose and friable. There are often voids 
between clasts in gravel deposits (an open-work structure).

o  Sizes of individual sediment grains are generally characterized 
by a log-normal frequency distribution. Particles can be of a wide 
range of sizes.  Clasts are usually rounded (see fig. 2 in 
Attachment 2). Sediment can be moderately to poorly sorted, 
particularly for flash floods  (see fig. 4 in Attachment 2).

o  Sediment usually is deposited as bars, fans, or sheets.

o  Sediment tends to be deposited on the inside of meander bends.

o  Channels have large width-to-depth ratios.

Deposits usually associated with debris flows:

o  Deposits usually are massive and unstratified.  Weak 
imbrication is sometimes present.  Inverse grading often is found 
near the base of deposit (see fig. 5 in Attachment 1 and figures 1 
and 3 in Attachment 2).

o  Beds are typically thick; 0.3 to 1 or 2 meters (1 m equals 3.28 

o  Deposits often consist of muddy, sandy gravel or muddy, 
gravelly sand.  Some mud--silt or clay--is nearly always present.  
Grains are typically angular but can include some rounded clasts 
picked up from the channel bottom.

o  Deposits typically are compact and difficult to dig out.  There 
are seldom voids between gravel clasts in the deposits.

o  Sediment usually is deposited as marginal levees or fronts 
(snouts) of very coarse material.  The snout is usually steep and 
lobate in form and contains a large concentration of boulders (see 
figure 5-E in Attachment 2).

o  Channels tend to be semicircular or even "U"-shaped.

o  Vegetation in the channel can sustain great damage.  Damage to 
vegetation may be minimal at the edges of the flow (see figure 5 
in Attachment 2).

o  Because the water-sediment mixture has some strength, sediment 
tends to be "pushed" to the outside of meander bends where it may 
be deposited.  This is the opposite of water floods where sediment 
tends to be deposited on the inside of meander bends.

All personnel responsible for investigating floods in mountainous 
basins need to be aware of the importance of correctly identifying 
flow processes and should have or be given adequate training and 
experience in flow-process identification using geomorphic and 
sedimentological evidence.  Training classes that discuss 
indirect-discharge methods sponsored by the National Training 
Center and Regional Offices now include instruction on ways to 
correctly identify flow processes.  Personnel are encouraged to 
take advantage of this training.  The Office of Surface Water and 
the Regional Surface Water Specialists are available to answer 
questions about processes associated with any flood event.

Glossary of terms used in this memo:

(Except where noted, definitions are from "Glossary of Geology" by 
Bates and Jackson (1987).

Alluvial fans--low, outspread, relatively flat to gently sloping 
mass of loose rock material, shaped like an open fan or a segment 
of a cone, deposited by a stream at the place where it issues from 
a narrow mountain valley.

Bar (streams)--ridgelike accumulation of sand, gravel, or other 
alluvial material formed in the channel, along the banks, or at 
the mouth, of a stream where a decrease in velocity induces 

Bed--informal term for strata that are incompletly known (after 
Bates and Jackson, 1980)

Clast--individual constituent, grain, or fragment of a sediment or 
rock, produced by the mechanical weathering (disintegration) of a 
larger rock mass.

Debris flow--flows in which solid particles and water move 
together as a single viscoplastic body (after Johnson, 1970).

Detrital sediment--sediment formed by the accumulation of 
detritus, especially that derived from pre-existing rocks and 
transported to the place of deposition.

Friable--said of a rock or mineral that crumbles naturally or is 
easily broken, pulverized, or reduced to powder, such as a soft or 
poorly cemented sandstone.

Hyperconcentrated flows--flows with concentrations greater than 40 
percent by weight (20 percent by volume)(after Beverage and 
Culbertson, 1964).

Imbrication--sedimentary fabric characterized by disk-shaped or 
elongated clasts dipping in a preferred direction at an angle to 
the bedding.

Inverse grading (reverse grading)--type of bedding that displays 
an increase in grain size with distance up from the base.

Lobate--having or consisting of a long tongue-like projection.

Newtonian fluid--substance in which the rate of shear strain is 
proportional to the shear stress.

Rheology--the study of the deformation and flow of matter

Stratified--formed, arranged, or laid down in layers or strata.

Sheet (deposits)--deposit that is generally stratiform, more or 
less horizontal, and areally extensive relative to its thickness


Bates R.L. and Jackson, J.A., 1980, Glossary of Geology, Second 
Edition:  American Geological Institute, Alexandria, Virginia,
749 p.

Bates R.L. and Jackson, J.A., 1987, Glossary of Geology, Third 
Edition:  American Geological Institute, Alexandria, Virginia,
788 p.

Beverage, J.P. and Culbertson, J.K., 1964, Hyperconcentrations of 
suspended sediment:  Journal of Hydraulics Division, American 
Society of Civil Engineers, v. 90(HY6), p. 117-128.

Costa, J.E., 1988, Rheologic, Geomorphic, and Sedimentologic 
Differentiation of Water Flood, Hyperconcentrated Flows, and 
Debris Flows, in Flood Geomorphology edited by V.R. Baker, R.C. 
Kochel and P.C. Patton:  John Wiley and Sons, Inc., New York,
p. 113-122.

Costa, J.E. and Jarrett, R.D., 1981, Debris flows in small 
mountain stream channels of Colorado and their hydrologic 
implications:  Bulletin of the Association of Engineering Geology, 
v. 18, p. 309-322.

Costa, J.E. and Williams, G.P., 1984, Open-File Report 84-606, 
Debris-flow Dynamics, 22.5-minute videotape.

Johnson, A.M., 1970, Physical Processes in Geology:  Freeman and 
Cooper, San Francisco.

                                 Charles W. Boning
                                 Chief, Office of Surface Water